bgl_named_params<Param, Type, Rest>

Many of the Boost.Graph algorithms have a long list of parameters,
most of which have default values. This causes several problems.
First, C++ does not provide a mechanism for handling default
parameters of template functions. However, this can be overcome by
creating multiply version of an algorithm with different numbers of
parameters with each version providing defaults for some subset of
the parameters. This is the approach used in previous versions of
Boost.Graph. This solution is still unsatisfactory for several
reasons:

The defaults for parameters can only been used in a particular
order. If the ordering of the defaults does not fit the users situation
he or she has to resort to providing all the parameters.

Since the list of parameters is long, it is easy to forget
the ordering.

A better solution is provided by bgl_named_params. This class
allows users to provide parameters is any order, and matches
arguments to parameters based on parameter names.

The following code shows an example of calling
bellman_ford_shortest_paths using the named parameter
technique. Each of the arguments is passed to a function whose name
indicates which parameter the argument is for. Each of the named
parameters is separated by a period, not a comma.

The order in which the arguments are provided does not matter as
long as they are matched with the correct parameter function. Here is
an call to bellman_ford_shortest_paths that is equivalent to
the one above.